1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine airfoil with a spar and shell construction.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an industrial gas turbine (IGT) engine, compresses air that is then burned with a fuel to produce a high temperature gas flow, which is then passed through a turbine having multiple rows or stages or stator vanes and rotor blades to power an aircraft or, in the case of the IGT, drive an electric generator. It is well known in the art of gas turbine engine design that the efficiency of the engine can be increased by passing a higher gas flow temperature through the turbine. However, the turbine inlet temperature is limited by the material properties of the turbine, especially for the first stage airfoils since these are exposed to the highest temperature gas flow. As the gas flow passes through the various stages of the turbine, the temperature decreases as the energy is extracted by the rotor blades.
Another method of increasing the turbine inlet temperature is to provide more effective cooling of the airfoils. Complex internal and external cooling circuits or designs have been proposed using a combination of internal convection and impingement cooling along with external film cooling to transfer heat away from the metal and form a layer of protective air to limit thermal heat transfer to the metal airfoil surface. However, since the pressurized air used for the airfoil cooling is bled off from the compressor, this bled off air decreases the efficiency of the engine because the work required to compress the air is not used for power production. It is therefore wasted energy as far as producing useful work in the turbine.
Recently, airfoil designers have proposed a new air cooled turbine rotor blade or stator vane design that is referred to as a spar and shell airfoil. U.S. Pat. No. 7,080,971 issued to Wilson et al. on Jul. 25, 2006 and entitled COOLED TURBINE SPAR SHELL BLADE CONSTRUCTION discloses one of these latest airfoils, the entire disclosure being incorporated herein by reference. The spar and shell construction allows for the use of a shell that can be made from an exotic high temperature alloy or material such as tungsten, molybdenum or columbium that could not be used in the prior art investment casting blades or vanes. Airfoils made from the investment casting technique are formed from nickel super-alloys and as a single piece with the internal cooling circuitry cast into the airfoil. Film cooling holes are then drilled after the airfoil has been cast. Without much improvement in the cooling circuitry of these investment cast nickel super-alloy airfoils, the operating temperature is about at its upper limit.
Thus, these new spar and shell airfoils will allow for the shell to be formed from the exotic high temperature materials because the shell can be formed using a wire EDM process to form a thin wall shell, and then the shell is supported by a spar to form the blade or vane. The exotic high temperature metals such as tungsten, molybdenum or columbium cannot be cast using the investment casting process because of their very high melting temperatures. However, thin walled shells can be formed using the wire EDM process. With a spar and shell airfoil having a shell made from one of these materials, the operating temperature can be increased way beyond the maximum temperature for an investment cast airfoil. Thus, the engine turbine inlet temperature can be increased and the engine efficiency increased.
One major problem with these new spar and shell airfoils that the applicants have discovered is that the shell and the spar have high thermal stress loads formed due to the large temperature differences. The shell is exposed to the high temperature gas flow while the spar, which can be made from the investment cast materials, is cooled with cooling air so that the temperature is much lower than the shell. If the shell is rigidly secured to the spar, the temperature difference will produce high thermal stress loads on ribs that connect the shell to the spar. A number of ribs are required to hold the thin shell wall to the spar when a high cooling air pressure is formed between the shell and the spar that tends to push the shell wall away from the spar. Thus, the ribs are used to hold the thin shell wall to the spar so that high pressure cooling air can be used between these two surfaces. If the ribs are rigidly fixed to the spar and the shell, then the high thermal stress loads will produce cracks in the ribs. U.S. Pat. No. 7,247,002 issued to Albrecht et al. on Jul. 24, 2007 and entitled LAMELLATE CMC STRUCTURE WITH INTERLOCK TO METALLIC SUPPORT STRUCTURE shows a composite turbine component with a ceramic shell secured to a metallic spar in which individual lamellae are supported directly by the support structure via cooperating interlock features formed on the lamella and on the support structure respectively. Mating load-transferring surfaces of the interlock features are disposed in a plane oblique to local axes of thermal growth in order to accommodate differential thermal expansion there between with delta alpha zero expansion. This design will allow for differential thermal growths along the airfoil spanwise (radial) direction parallel to the interlocking features, but not in a direction perpendicular to this such as along a direction parallel to the chordwise direction (the line from the leading edge to the trailing edge through the center of the airfoil cross section in the plane of FIG. 1 of the Albrecht patent).